1. Field of the Invention
The invention relates generally to a method of forming a semi-insulating silicon carbide (SiC) single crystal. More particularly, the invention relates to a method of forming a semi-insulating SiC single crystal without increasing the concentration of intrinsic defects associated with the SiC.
2. Description of the Related Art
A semiconductor is a solid crystalline material whose electrical conductivity is intermediate that of a conductor and an insulator. Semiconductors exhibit resistivity between about 10−3 ohm-cm and 109 ohm-cm. “Semi-insulating” semiconductors constitute a special class of semiconductors formed from materials having a resistivity of at least 103 ohm-cm. Semi-insulating semiconductors include, for example, SiC, GaAs, GaP, InP, CdTe, GaN, and AlN, and find application across a broad range of applications including microwave devices and optoelectronics.
In order to achieve a resistivity consistent with semi-insulating behavior, shallow donor and/or acceptor levels originating from impurities in the semiconductor material must be eliminated or compensated. Two general approaches to the compensation of shallow donor/acceptor levels resulting from impurities in a semiconductor material have been identified; one relies on the intentional doping of the semiconductor material with one or more transition or heavy metal impurities, and the other relies on the formation of intrinsic (naturally occurring) defects in the semiconductor material. (See, for example, D. Nolte, J. Appl. Physics, Vol. 85, No. 9, pp. 6259-89 (1999)).
The first of these two general approaches is described, for example, in U.S. Pat. No. 5,611,955 in which a deep level dopant is introduced into a process used to form a SiC single crystal. A transition metal such as vanadium is proposed as a deep level dopant. The second of the two general approaches is disclosed, for example, in U.S. Pat. No. 6,218,680 in which intrinsic point defects are intentionally and additionally introduced into the semiconductor material during its formation process. Both of these approaches can effectively compensate for the dominating type of shallow acceptors or shallow donors. However, both approaches suffer from certain drawbacks.
For example, proponents of the second approach recognize the well-established fact that production of a semi-insulating SiC single with heavy or transition metal doping can in certain circumstances lead to deterioration in crystal quality and/or low process yield due to non-uniform dopant profiles, and/or second phase or segregation effects. (See, for example, J. R. Jenny, Appl. Physics Letters, Vol. 68, No. 14, p. 1963 (1996)). Thus, the second approach seeks to overcome these complications by the use of intrinsic point defects acting as deep levels compensating the free carriers introduced into the SiC single crystal by shallow level acceptors or donors. Intrinsic point defects include vacancies, antisites, interstitials and similar imperfections, as well as pairs, complexes, and precipitates of these intrinsic point defect types, that naturally occur in the lattice structure of the semiconductor material.
In turn, however, the second approach, which is characterized by an increased number of intrinsic defects, is not without its own problems. For example, U.S. Published Patent Application No. 2003/0079676, incorporated herein by reference, recognizes that certain intrinsic point defects are thermally unstable, and if used in a semi-insulating SiC substrate, the resistivity of the substrate may not be well controlled across a range of operating conditions or throughout a sequence of processing steps applied to the substrate. For example, some annealing steps applied to the SiC substrate may remove the silicon vacancies relied upon, in part, to produce the substrate's semi-insulating behavior. Additionally, traps formed by the intrinsic point defects in the SiC substrate can cause collapse of drain-source currents for certain devices formed on the SiC substrate, such as Schottky gate field effect transistors.
Accordingly, U.S. Published Patent Application No. 2003/0079676 seeks to balance a suitable combination of deep levels, both acceptor and donor, as well as intrinsic and extrinsic in nature. Supposedly, such a combination of intrinsic defects and extrinsic defects can be adjusted, such that the effect on electrical behavior of the SiC resulting from the extrinsic defects is balanced or compensated by an opposite effect from the intrinsic defects. Unfortunately, this approach requires a level of deep level dopants, that while less than the level of the first approach referenced above, is yet too high.
In yet another conventional approach, U.S. Pat. No. 5,856,231 suggests that nitrogen impurities having a concentration less than 5×1016 cm−3 may be “overcompensated” by the addition of a trivalent element (preferably aluminum). Furthermore, this approach suggests adding a transition element (preferably vanadium) having, at least in SiC, donor levels in the center of the energy gap in order to compensate for excess acceptor levels. As such, this approach requires the use of both shallow level and deep level dopants to obtain semi-insulating behavior since aluminum is intentionally added in overcompensating amounts higher than that needed for the compensation of nitrogen impurities.